Our evidence supports the argument that suburbanized employment -- or sprawl -- is
associated with shorter distance commutes on average. This is not to say that commutes
are shortening as cities expand their footprint; indeed, they seem to be slowly lengthening
(Kahn, 2000). Other factors, such as rising incomes, are apparently generating more
and longer trips to work. Contrary to much conventional folklore, however, the marginal
effect of job suburbanization appears to be to bring jobs and workers closer. Put
another way, the average commute would be longer still if jobs were not suburbanizing.

A closer look reveals that this is not a transparent outcome. There are strong differences
by industry, for example. The suburbanization of construction, wholesale, and service
employment is associated with shorter commutes, while manufacturing and finance deconcentration
(weakly) explain longer commutes. These results may reflect a combination of industry
agglomeration effects, differential job location stability by industry, and historical
transitions.

What does this mean? First, our study is exploratory and thus preliminary. Many loose
ends remain, suggesting numerous ways to refine and extend our understanding of these
relationships. On the one hand, the AHS data do not allow U.S. to test how commute
duration has changed. This ignores the substantial role of congestion in urban form
and behavior debates (Solow, 1973; Wheaton, 1998). Other evidence suggests that congestion
may be lower in outlying areas, so the dispersal of employment to outlying counties
within metropolitan areas may actually reduce commute duration more than it does
commute distance. Alternatively, if job dispersal to outlying counties is associated
with a higher amount of non-work travel by affluent households living in those areas,
commute duration could increase despite a slight decline in commute distance. This
is because peak period travel duration is only partially accounted for by work trips;
non-work trips make up a growing majority of trips made during peak periods (Giuliano,
1991).

In addition, while these results partially support the Crane (1996) hypothesis that
commutes are longer for individuals with greater uncertainty about their future job
locations and higher moving costs, our data do not permit us to explore that conjecture
in detail.

Finally, even if further study provides consistent support for the "sprawl shortens
commutes" story, the public policy implications require more discussion. On the one
hand, there is the issue of whether, on net, jobs follow workers to the suburbs or
vice versa. The evidence is mixed on either count, suggesting the simultaneity of
this process is challenging to nail down empirically. If jobs returned to the city,
is it possible that workers would follow suit? On the other hand, neither the influence
of urban form on travel behavior nor the merits of concentrated versus dispersed
urban growth are well understood. The former is yet another complex set of nuanced
behaviors awaiting better data and empirical strategies (Boarnet and Crane, 2001).
Regarding the latter, we do not know how the social and economic costs of sprawl,
however measured, compare with their benefits (Crane and Greenstein, 2002). A key
purpose of our study is to explore the underlying behaviors needed for both kinds
of evaluations.